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I have data formatted like {{a,b},{c,d},{e,f}} where each entry is an integer. I need to turn this in to a sparse array with entries {a,1}->1, {b,1}->1, {c,2}->1, ... {f,3}->1 and so on. In other words, if $x$ is an integer in the $j$th list, we need coordinate $(x,j)$ of the sparse matrix to be $1$.

How should I accomplish this?

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  • $\begingroup$ You might want to look up IncidenceMatrix[]. $\endgroup$ – J. M. will be back soon Jun 30 '16 at 4:55
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l = {{a, b}, {c, d}, {e, f}};
Flatten[MapIndexed[{#1, #2[[1]]} -> 1 &, l, {2}], 1]

    (*{{a, 1} -> 1, {b, 1} -> 1, {c, 2} -> 1, {d, 2} -> 1, {e, 3} -> 
  1, {f, 3} -> 1}*)
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